Background: Dental patients routinely complete a medical questionnaire and have an oral interview during their initial visit, but may have undiagnosed systemic problems which can affect their dental treatment. Methods: Thirty-nine consecutive patients referred for a periodontal evaluation completed a medical questionnaire and an oral interview. They were referred to a hospital laboratory for an urinalysis, complete blood count, and a standard blood chemistry panel. Results: The self-reported medical history responses were compared with the laboratory data and several abnormalities were noted. Abnormal levels were found with cholesterol, (26/39 patients, 67%); triglycerides, (13/39, 33%); glucose, (6/39, 15%); eosinophils, (18/39, 46%); and monocytes, (10/39, 26%). Smokers (17/39, 44%) had a higher number of abnormal levels or percentages of cholesterol, triglycerides, basophils, lymphocytes, eosinophils, and monocytes. Gender differences were seen in elevated triglyceride levels, abnormal aspartate aminotransferase (AST) and alanine aminotransferase (ALT), and self-reported cardiovascular disease. Conclusions: This study demonstrated that many patients are unaware of their current medical status and a significant number had undiagnosed abnormalities.